Augmentation of Chains. 489 Flexible links are called tension elements; and fluid con- nections, as between boiler and engine, or accumulator and machine, are termed pressure elements, but the latter are always connected to lower pairs. A pump is kinematically the same as a ratchet, the valves being equivalent to pawls (see Fig. 450). Augmentation of Chains is the multiplication of parts, for convenience or the reduction of friction. Trains of gearing, and anti-friction rollers (Fig. 451), are examples. Summing up, mechanism may be divided into simple chains, formed of rigid or flexible links, which are again united by higher or lower pairs ; and all chains must be dosed, either by the chain or by external force.* LIST OF KINEMATIC CHAINS. Lower ( *• Crank chains : pairing, t 2, Screw chains : High [3- Pulley chains : and low 4. Wheel chains: 5. Cam chains: 1 Sliding and turning and screw J pairs. Tension and pressure elements. Uniform motion. Variable motion. pairing. ^ 5. Ratchet chains: Intermittent motion. A driving and working end are recognised in each of thesey corresponding to P and W respectively, and the Velocity Ratio of P and W in Kinematic Chains will now be investigated graphically. Considering the instan- Friction closure is one form of force closure.